(a) Technical Field
The present invention relates to a method of adjusting the characteristics of a clutch in a hybrid electric vehicle, which is capable of maintaining the characteristics of the clutch at an appropriate level while reducing the frequency of the adjustment of the clutch characteristics, thereby resolving annoyance a driver or passengers may feel when the adjustment is frequently made.
(b) Background Art
A hybrid vehicle basically refers to a vehicle that uses two or more power sources to propel the vehicle in a fuel-efficient manner. Mostly, the term ‘hybrid vehicle’ is used for denoting a hybrid electric vehicle (HEV) that includes an internal combustion engine powered by fuel and an electric motor powered by a battery.
The hybrid vehicle is driven by using electric energy from the battery and mechanical energy from the engine. In the hybrid vehicle, both the electric motor and the engine may be operated optimally and regenerative braking may be employed as well. Accordingly, high energy efficiency can be achieved.
FIG. 1 schematically illustrates a powertrain (drivetrain) of a parallel hybrid vehicle. The parallel hybrid vehicle comprises an engine 10 as a power source, a motor-generator (MG) 20, a clutch 12 situated between the engine 10 and the motor-generator 20, and an automatic transmission 30 to which the output end of the motor-generator 20 is coupled.
The parallel hybrid vehicle is driven in three driving modes: HEV mode, EG mode and EV mode. In HEV mode, both the engine 10 and the motor-generator 20 are used in combination to propel the vehicle. In EG mode, only power from the engine is used. In EV mode, only power from the motor-generator 20 is used. The driving mode can be selected according to operation (engagement/disengagement) of the clutch 12, which is controlled by, e.g., a hydraulic device. Namely, the clutch 12 is engaged (closed) in EG mode and HEV mode while it is disengaged (opened) in EV mode.
In practical, during low speed driving or at starting of the vehicle, the motor-generator 20, which exhibits maximum torque at low RPM, is used to drive the vehicle When the vehicle enters into steady speed cruising state, the engine 10 is started by means of an integrated starter/generator (ISG) 40 so that the outputs from the engine 10 and the motor-generator 20 can be utilized in a combined manner. The outputs from the engine 10 and the motor-generator 20 are transmitted to the automatic transmission 30, which then provides desired rotation speed to driven wheels through a drive shaft 50.
More specifically, during low speed driving or at starting of the vehicle, the clutch 12 is disengaged and the motor-generator 20 thus becomes a sole power source to drive the vehicle. Since engine torque is minimal at low RPM while motor-generator torque is maximal at low RPM, fuel efficiency can be improved by using the motor-generator 20 to complement the torque of the engine 10 at low RPM.
When a driving mode is changed from EV mode to EG or HEV mode, i.e. as the clutch 12 become engaged, awkward clutch engagement may often occur with undesirable shock if clutch control is performed without considering the characteristics of the clutch 12 such as thrust against a clutch disc, slip rate, torque transmission, relationship between temperature and kinds of oil.
Japanese Patent No. 3657902 discloses a method for controlling the operation of a clutch in a hybrid vehicle by feedback control. The method, however, involves undesirable vibration occurring during clutch engagement. Further, Japanese Patent No. 3657902 discloses a method of learning and adjusting the characteristics of a clutch when clutch slip is available such as in the event of vehicle deceleration. The adjustment, however, is still insufficient.
On the other hand, Korean Patent Application No. 10-2007-0128676 filed by the present inventors discloses a method of adjusting the characteristics of a clutch in a hybrid vehicle, which comprises: idle-rotating the engine by transmitting power from the motor-generator with the clutch in half-clutch state in a state where an output to the transmission is blocked and the engine is in a fuel-cut state; estimating a clutch transmitting torque based on the torque of the motor-generator when rotating speeds of the engine and the motor-generator become constant; and adjusting the characteristics of the clutch based on the estimated clutch transmitting torque. This method, still, can be applied only when the transmission is in neutral (non operation) state. For this reason, it is hard to perform such adjustment frequently.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.